This is how I do it and I use this cheat almost every time I play the Sims.
Did you ever use the old cheat for Sims 2 "boolprop testingcheatsenabled true" ? It allows you to move almost all your sims' bars (i.e. needs, happiness, fitness, relationships, skills, and even school grades (I think)).

So for the Sims 3, the cheat is now: testingcheatsenabled true

If you want to take this cheat off, replace "true" with "false".
You have to be in the neighborhood to put in this cheat (I think, not sure for Sims 3). So go there, open the cheat box (Ctrl+Shift+C), and type that cheat in.

Then you can mess around with their skills if you want. Here's the thing, though, about the Sims 3 which sort of sucks. You have to get the first level of a skill achieved before you can up the skill level (unlike in Sims 2 where the skills were still shown even if they were at level 0)

sims 2 was soooooo much better cause when you put in the cheat
boolprop testingcheatsenabled true
all you them had to do was shift click and drag up the sims skills but the sims 3 doesnt let you do that

Well I know one way.(Game console and more than 1 sim needed) First, make your sim watch a channel that boosts you skills and then switch to another sim and make them (or they) play the console, Even though the TV is emitting video games, your sim learns a skill. Useful when short on time for work, party,etc. Warning!!! Will not be up if you do anything else like cooking, fishing, gardening etc. Trust me it works 100% just try it and see. http://images.google.com/imgres?imgurl=http:

There is no such cheat in the game itself, but you can download the so called ''Book of Talent'' at Mod the Sims. It's very easy to use and totally safe. Read the instructions before you download it. If you don't know how to store .package files, there's an installing guide at this site as well.
Links:
For the book of talent > http://www.modthesims.info/download.php?t=383662
For help > http://www.modthesims.info/wiki.php?title=Sims_3:Installing_ Package_Files

If you want get skills faster just type in cheatbox (ctrl+shift+c) testingcheatsenabled true and holding shift press on the mailbox and click on needs static and your sims will always be happy then you can paint all time or read or run or write.